The present invention relates to an exhaust gas purification device of an engine for a vehicle, and more particularly, to an exhaust gas purification device of an engine for a vehicle which comprises a filter member disposed in an exhaust passage of the engine to trap exhaust particulates in an exhaust gas.
Conventionally, for example, a diesel engine is equipped with a filter member, so-called a particulate filter, which is disposed in an exhaust passage so as not to emit exhaust particulates of carbon or the like in its exhaust gas to an atmosphere.
In the engine with such a particulate filter, it is required to burn trapped particulates by the particulate filter and thereby regenerate the filter when the amount of tapped particulates has reached to a maximum value to be trapped at the filter. Accordingly, for example, Japanese Patent Laid-Open Publication No. 4-86319 discloses a device in which there is provided a manual switch operative to regenerate such a particulate filter and regeneration of particular filter is performed by increasing the amount of fuel injection of an engine for a while to raise an engine idle speed and thereby burn exhaust particulates trapped by the filter when the manual switch is switched on in a state of vehicle stop.
Herein, the above-described regeneration control is configured such that when it is determined that the state of the vehicle changes from the stop state to a driving state by detecting, for example, pressing of an accelerator pedal (so-called racing operation), or changing of shift range from its stop ranges (P range, N range) to its driving range (D range), the engine speed is so controlled as to return (decrease) to its specified normal idle speed, not to the above-described raised higher speed, because raising the engine speed to such higher speed may cause some problems in a start feeling for a vehicle driver.
However, the inventors of the present invention found out another problem that if the engine speed which has been raised once is returned to its specified normal idle speed soon after the above-described driver's intention to start the vehicle is detected during the regeneration of the particulate filter, the temperature of the particulate filer may increase too rapidly, thereby deteriorating the durability of the particulate filter.
Namely, the temperature of the particulate filter increases due to burning of the exhaust particulates during the regeneration, and if such returning of the engine speed to the specified normal idle speed is executed during this state where the temperature of the particulate filter is increasing, the amount of exhaust gas flowing into the particulate filter comes to decrease. Thus, a cooling function of particulate filter by the exhaust gas (a heat exchange between the exhaust gas and the particulate filter), i.e., the exhaust gas flowing in cools the particulate filter itself, may be reduced. As a result, the temperature of the particulate filter may increase too rapidly.
Accordingly, the inventors of the present invention considered countermeasures to this. Namely, when it is determined that the state of the vehicle changes from the stop state to the driving state during the regeneration of the particulate filter, the engine speed is so controlled as not to return to the specified normal idle speed, but to return to a specified engine speed which is slightly higher than the specified normal idle speed in order to suppress such improperly rapid temperature increasing of the particulate filter, avoiding any problem of start feeling of the vehicle.
However, there may occur further another problem of an improper fuel efficiency if the above-described engine speed control that the engine speed is returned to the specified engine speed slightly higher than the specified normal idle speed is always performed regardless of the temperature of the particulate filter. Namely, in the event that the temperature of the filter member is relatively low and thereby an improper influence of the above-described temperature increasing of particular filter is small, such controlling of the engine speed to the specified engine speed slightly higher than the specified normal idle speed may deteriorate the fuel efficiency.